I have a lifelong love for the British radial engines and in particular the Bristol Centaurus. This is particularly so following a 50 minute cross country flight in a Hawker Sea Fury from Madera to Chino in California in the summer of 1980. The aircraft was owned and operated by the late Frank Sanders and its condition was a testimony to his dedicaton to the restoration and operation of vintage aircraft.

I have read extensively of the design and development of the sleeve valve engines produced by Bristol, Napier and Rolls Royce. In all of the articles I have read I have noted that the cylinder heads are always described as JUNK heads,--- WHY ????? What is the derivation of this description. Can anyone enlighten me ?

The term "junk head" goes back to the 19th century and the steam age. However, I still don't konw why they are called junk heads. I served an apprenticeship in the early 60s and that's where I first ran into this term except it was referring to steam engines then. (Yep, I'm that old..!!)

"Junk Head" is a contraction of the term, "junk-ring cylinder head" indicating a type of cylinder head which contains sealing rings somewhere in its structure.
In olden days, "junk" was a form of building material made from weaving the stems and leaves of rushes and reeds, many of which were from the Juncus botanical group. (L. juncus=a reed). Junk was used with wood and stone to fill gaps and seal dwellings from the weather.
Later, in sailing ships, it was common practice to cut up old rope into lengths, unpick it and weave it roughly together for use in patching and sealing ship's timbers. This material inherited the name "junk". When mixed with tar and used for caulking, it was called "oakum".
When junk was wrapped around the end of a stick for use in ramming gunpowder into ship's cannons, the junk layer was held in place by steel bands, hence the term, "junk rings" and the subsequent association with the function of sealing. The dictionary also defines "junk ring" as, "a metal ring for keeping piston packing in place". This refers to useage in steam engines.
So, there is no such thing as a "junk head" per se - the term is only an abbreviation. Neither is there any association with the idea of rubbish or worthlessness in this context.
Having said all that, it should be appreciated that the application of the terms "junk head" or "junk ring" to any sleeve-valve engine is completely inappropriate and erroneous. The purpose of a junk ring is to hold some form of piston packing material (ie, a junk layer) in place. Such packing layers were never used in sleeve-valve engines, either in the cylinder head or the cylinder itself.
It would be interesting to know who first coined the term; whoever it was, got it wrong!